Scanner system having a dual trace spinner

ABSTRACT

The present invention encompasses a scanning optical system such as internal drum photoplotters that has a raster scanner for advancing a circularly polarized optical beam across a substrate surface in a first direction to form a scan line and for advancing the optical beam in a second direction substantially perpendicular to the first direction. There is a curved platen for receiving a substrate and an apparatus for switching the optical beam polarization between first and second directions in response to polarization control signals;. An encoder generates signals indicative of the position of the optical beam along a current scan line. There is a controller that receives the encoder signals and generates the advancement signals and the modulator control signals. The controller further provides the optical beam polarization switching signals in dependence on the encoder signals such that the optical beam polarization is switched after the completion of the current scan line. The system is characterized by a spinner which uses polarization switching to generate two scan lines per spinner rotation.

This is a continuation-in-part of application Ser. No. 08/601,422, filedon Feb. 14, 1996, now abandoned, and Ser. No. 08/311,573 filed on Sep.23, 1994, now abandoned.

TECHNICAL FIELD

The present invention relates to scanners and imagers in general and,more particularly, to scanners in having a dual scan spinner for anenhanced operational efficiency.

CROSS REFERENCE TO RELATED APPLICATIONS

Some of the subject matter herein is disclosed and claimed in thefollowing U.S. patents, all of which are incorporated herein byreference.

U.S. Pat. No. 5,291,392 entitled "Method And Apparatus For Enhancing TheAccuracy Of Scanner Systems";

U.S. Pat. No. 3,555,254, entitled "Error Correcting System And MethodFor Use With Plotters, Machine Tools And The Like";

U.S. Pat. No. 4,851,656 entitled "Method And Apparatus For EnhancingOptical Photoplotter Accuracy".

BACKGROUND OF THE INVENTION

Raster scan photoplotters or imagers having both planar and internaldrum design are known in the art. These devices are used in thefabrication of printed circuit boards. Conversely, scanners which readdata from a substrate have similar geometries. Planar photoplotters suchas disclosed and claimed in U.S. Pat. No. 4,851,656 have a planarsurface for receiving a substrate. An optical exposure head is locatedon a movable gantry apparatus and is rastered above the substrate duringexposure. Internal drum photoplotters are characterized by asubstantially cylindrical surface portion which receives the substrate.The exposure beam emanates from an optical exposure head and is scannedacross the substrate by a rotating spinner. The optical exposure head isindexed along the longitudinal axis of the cylinder to complete thesubstrate exposure. Internal drum raster photoplotters of the typedisclosed in U.S. Pat. No. 5,291,392 have inherent advantages overplanar type scanners, including simplicity of design and lower costs.

An exemplary internal drum laser raster imager, the Crescent 42manufactured by Gerber Scientific, Inc. of South Windsor, Conn., has aninternal drum that utilizes a 180° curved surface to receive thesubstrate. It also has a spinner centered on a longitudinal drum axis.With this configuration, one rotation of the spinner with its nominal45° scan mirror produces one scan line; yielding a duty cycle of about50%. As the raster image processing or "RIPing" technology oftransferring data and thereafter interpreting it progresses, so does thedesire to image faster. However, there are difficulties in advancing theimaging speed of internal drum imagers. The spinner itself is limited toa speed in the range of 20,000 to 24,000 RPM by the air bearing/motortechnology and mirror deformation considerations. Another avenue ofinquiry involves the use of multiple beams, However, a multiple beamapproach is highly difficult to implement due to the internal drumscanning geometry which produces an undesirable rotation in the imageplane of multiple beams so that they no longer lie in a plane withrespect to the motion axes. Solving this problem requires the additionof a costly and complicated rotating prism assembly which must besynchronized to the spinner.

A further, related issue is the desire to increase the temporalefficiency of the scanner or imager. As noted, prior art systems arelimited to a maximum 50% duty cycle. Internal drum imagers can bemanufactured with higher angular utilization (i.e. 270°) with higherduty cycle but they add complexity for material handling. A limited dutycycle is undesirable from two respects. First, the lower the duty cycle,the faster the video electronics must be for an equivalent scan rate.Secondly, for systems such as computer-to-plate and direct imaging ofprinted circuit boards, there can be an exposure limitation. A higherduty cycle improves the system's ability to expose the substrate media.

Earlier efforts to improve the overall throughput of imaging or scannersystems include the device disclosed in U.S. Pat. No. 5,187,606 to Kondoet al. The '606 device shows a scanning optical apparatus that has alight source for emitting a light beam and a deflector, such as arotating polygonal mirror, with a plurality of mirror surfaces fordeflecting the light beam. Each mirror surface of the polygonal mirrorhas a pair of reflecting surfaces inclined toward the center axis ofrotation of the polygonal mirror and orthogonal to each other. There isa fixed reflecting mirror arranged in an opposed relationship with oneof the pair of reflecting surfaces so that the light beam deflected bythe reflector is reflected, to be returned to the deflector again. The'606 system is used to increase the scanning angle of the laser beam totwice the width as compared to that of conventional polygonal mirrors,thereby increasing the speed of scan without increasing the rotationalspeed of the polygonal mirror. U.S. Pat. 4,445,126 to Tsukada disclosesan image forming apparatus in which recording medium is scanned with aplurality of light beams. The '126 apparatus includes a beam generatorfor generating a plurality of light beams and presenting themsimultaneously to a facet of rotating polygonal mirror. The purpose ofthe '126 apparatus is to generate a plurality of scan lines at a giventime during operation.

An image recording device which relies on multiple beams is disclosed inU.S. Pat. Nos. 4,506,275 and 4,517,608 to Maeda et al. The Maeda et aldevice includes a recording unit for duplicating and recording halftoneimages on photosensitive material. The recording unit comprises anacousto-optic light modulating element including a plurality ofultrasonic wave exciting portions disposed side by side on a singleacousto-optic medium. The ultrasonic wave exciting portionsindependently modulate an incident light beam into a plurality ofmodulated light beams in response to image signals from aphotoelectrical scanning means. There is a scaled down optical systemwhich then reduces the diameter of the plurality of modulated lightbeams at a plurality of light transfer elements to transfer the lightbeams from the scaled down optical system to a focusing lens to beprojected onto a film in a recording cylinder. The system as set forthin the Maeda et al patents relies on a fixed scanning head. Thesubstrate is located on external surface of the rotating drum.

A multiple beam optical modulation system is disclosed in U.S. Pat. No.5,251,057. The '057 system is used in a raster output scanner thatemploys one original beam and a facet of a rotating polygon to generateto consecutive scan lines. The original beam is first separated into twobeams in a beam splitter. The resultant beams are polarized ninetydegrees apart, and directed to a modulator. The beams are a sufficientdistant apart so that the acousto-optic (a/o) modulator can modulateeach beam with a minimum of crosstalk. The output beams are put broughttogether to within one scan line separation by a beam recombinationdevice, which is a reversed beam splitter. The beams can be broughttogether to close proximity without optical interference because thebeams are polarized ninety degrees apart.

None of the systems disclosed by the prior art offer a doubling of scansystem efficiency nor is there found a system which achieves anyimprovement in throughput without extensive and cumbersome modificationsto system optics and electronics. It would be advantageous to have asystem for use with internal drum type scanners or photoplotters whichprovides two scans for each rotation of the system spinner. The presentinvention is drawn toward such a system.

SUMMARY OF INVENTION

An object of the present invention is to provide an optical spinner foruse with a photoplotter or scanner that provides two scan lines for eachrotation.

Another object of the invention is to provide a spinner of the forgoingtype that allows for approximately one hundred per cent duty cycleoperation.

Still another object of the present invention is to provide a system ofthe foregoing type in which the system throughput approximately doublesfor a given spinner rotation speed.

According to one aspect of the present invention, a scanning opticalsystem includes an optical source for generating a circularly polarizedoptical beam. There is a curved platen to receive a substrate and amodulator for providing optical modulation to the circularly polarizedoptical beam in response to received modulator control signals. A rasterscanner is responsive to advancement control signals and advances,relative to the substrate, the circularly polarized optical beam acrossthe substrate in a first direction forming a scan line. The rasterscanner also advances the circularly polarized optical beam relative tothe substrate in a second direction substantially perpendicular to thefirst direction displacing one scan line from another. There is also anapparatus for switching the optical beam polarization between first andsecond directions in response to polarization control signals. Anencoder generates signals indicative of the position of the circularlypolarized optical beam along a current scan line. A controller receivesthe encoder signals and generates the advancement signals and themodulator control signals. The controller further provides the opticalbeam polarization switching signals in dependence on the encoder signalssuch that the optical beam circular polarization is switched after thecompletion of the current scan line. The scanning optical system alsoincludes a spinner that receives the optical beam from the polarizationswitching apparatus. The spinner has an quarter wave plate that receivesthe circularly polarized optical beam and provides a linearly polarizedscan beam. A polarization sensitive beamsplitter reflects the linearlypolarized scan beam at an internal beamsplitter surface if the linearlypolarized scan beam is polarized in a first linear direction. A quarterwave plate receives the linearly polarized scan beam from thepolarization sensitive beamsplitter if the linearly polarized scan beamis polarized in a second linear direction orthogonal to the first lineardirection. The quarter wave plate rotates the second direction polarizedscan beam by ninety degrees as it transits the same. There is aretroreflector for returning the ninety degree rotated second directionpolarized scan beam through the quarter wave plate to the polarizationsensitive beamsplitter.

According to another aspect of the present invention, a spinner for usein a scanning optical system that has an apparatus for generating acircularly polarized optical beam, an apparatus for switching theoptical beam polarization between first and second directions, a curvedplaten for receiving a substrate and a raster scanner responsive tocontrol signals for advancing, relative to the substrate, the circularlypolarized optical beam across the substrate in a first direction forminga scan line and an encoder for generating signals indicative of theposition of the circularly polarized optical beam along the current scanline. The spinner includes an quarter wave plate for receiving thecircularly polarized optical beam and provides therefrom a linearlypolarized scan beam. There is a polarization sensitive beamspliterreflecting the linearly polarized scan beam at an internal beamsplittersurface if the linearly polarized scan beam is polarized in a firstlinear direction. A quarter wave plate receives the linearly polarizedscan beam from the polarization sensitive beamsplitter if the linearlypolarized scan beam is polarized in a second linear direction orthogonalto the first linear direction. The quarter wave plate rotates the seconddirection polarized scan beam by ninety degrees. A retroreflectorreceives and returns the ninety degree rotated second directionpolarized scan beam through the quarter wave plate to the polarizationsensitive beamsplitter.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified schematic illustration of a portion of aninternal drum raster imager system including a spinner provided inaccordance with the present invention.

FIG. 2 is a diagrammatic illustration of a portion of the system of FIG.1 showing a relationship between the scanned beam and the spinner.

FIG. 3 is a simplified schematic illustration showing an initial portionof a scan of an optical beam across the internal drum surface by a priorart photoplotter.

FIG. 4 is a simplified schematic illustration showing a final portion ofthe scan of FIG. 3.

FIG. 5 is a diagrammatic illustration showing the effective duty cycleof the photoplotter of FIG. 2.

FIG. 6 is a simplified schematic illustration of a spinner provided inaccordance with the present invention receiving a counterclockwise,circular polarized light beam.

FIG. 7 is a simplified schematic illustration of a spinner provided inaccordance with the present invention receiving a clockwise, circularpolarized light beam.

FIG. 8 is simplified schematic illustration of scan lines written byfirst and second scan beams not compensated for scan error.

FIG. 9 is a simplified schematic illustration of an alternativeembodiment of the present invention having a Roof Prism.

FIG. 10 is a simplified schematic illustration of another alternativeembodiment of the present invention having a Wollaston prism.

FIG. 11 is a sectioned illustration of the Wollaston prism of FIG. 10.

FIG. 12 is a sectioned illustration of the lens in the embodiment shownin FIG. 10.

FIG. 13 is a diagrammatic illustration detailing an alignment errorintroduced in the first scan beam in the system of FIG. 9.

FIG. 14 is a diagrammatic illustration showing effect of the alignmenterror illustrated in FIG. 13 in the second scan beam in the system ofFIG. 9.

FIG. 15 is a diagrammatic illustration showing effect of the alignmenterror in the second scan beam of the system of FIG. 9 with a mirrorsubstituted as an optical return.

FIG. 16 is a sectioned illustration of a spinner with a Rochon prismused as an alternative to the Wollaston prism of FIG. 10.

FIG. 17 is a diagrammatical illustration of an alternative embodimentfor the present invention characterized by simultaneous presentation ofdual scan beams.

FIG. 18 is simplified diagrammatic image of a modified Wollaston prismassembly used with an alternative embodiment of the present invention.

FIG. 19 is a simplified schematic illustration of a portion of themodified Wollaston system of FIG. 18 showing the path of a beam of firstpolarization.

FIG. 20 is a simplified schematic illustration of the modified Wollastonsystem of FIG. 18 showing the path of a beam of second polarization.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to both FIGS. 1 and 2, there is shown in simplifiedschematic form a portion of an internal drum raster photoplotter 10having an internal drum 12 with a surface 14 that comprises a portion ofa cylinder. The internal drum is carefully fabricated and must maintainthe cylindricity of the drum surface with great accuracy regardless ofvariations in environmental parameters such as temperature. To that endthe internal drum is a substantial structure preferably of cast aluminumwith a series of reinforcing ribs (not shown) spaced along an outsideperimeter.

The drum surface is adapted to receive a substrate and includes aplurality of holes 16 which communicate with a plurality of internalchannels 18 through which a vacuum is generated by conventionalapparatus not shown in the drawing. The vacuum is used to hold asubstrate 21 in place during the exposure process. Alternative methodscan be equivalently used to hold the substrate in place, includingelectrostatic and mechanical retention techniques.

The photoplotter also includes a rail 20 that has a carriage mountedraster scanner 22 for scanning an optical beam 24 about the substratesurface in response to command signals received from controller 26 in amanner detailed hereinafter. The raster scanner includes a linearencoder 28 for generating signals indicative of the position of theraster scanner as it moves along the rail. Also included is a fast scanapparatus 30, preferably comprised of a motor 32 and a spinner 34, forreceiving the optical beam at a mirror surface 35 from an optical beamsource, such as laser 36, and for exposing a series of scan lines 38 onthe substrate by rotating the spinner about a spin axis 40, typically at12,000 rpm. A rotary encoder 42 is included for generating signalsindicative of the angular position of the mirror surface during a scan.The optical beam is provided along the spin axis to be received at acentral point on the mirror surface.

FIG. 3 is a simplified schematic illustration of a portion of a priorart photoplotter 44. Shown in FIG. 3 is a first portion of a rotary drumsubstrate surface 46 which receives a beam of light 48 reflected from amirror surface of spinner 50. The spinner 50 is rotated about arotational axis 52 and advances the beam from right to left in thefigure. The spinner mirror surface is oriented at 45 degrees along thecentral axis of the internal drum which also corresponds to the opticalaxis along which the exposure beam traverses before presentation to thesubstrate surface. The mirror surface is oriented to the optical axis ofa laser beam and presents the beam directly to the surface. A fullrotation of the spinner will yield a laser beam presented to the entireinternal drum surface; both the section containing the substrate and theremainder thereof.

There is an initial spinner position 54 before which the beam wouldotherwise be presented above the internal raster drum substrate surface46 and therefore not to the substrate. FIG. 4 shows a second spinnerposition 56 subsequent to the initial position shown in FIG. 3 in whichthe beam is almost completely advanced across substrate. The rotationalextent of these two positions is displayed diagrammaticaly with respectto FIG. 5 by curve 58. Beyond spinner position 56, the spinner mustrotate around to its initial position shown in FIG. 3 before thecontroller can again present the modulated exposure beam for creating ascan line. In many scanners, the internal drum surface which receivesthe substrate extends only 165 degrees, much less than the practicalupper bound of 180 degrees. As a result, the duty cycle of prior artsystems is even less than 50%.

FIGS. 6 and 7 are simplified schematic drawings showing a spinner 60provided according to the present invention. The spinner allows for twoscan lines for each rotation. In the present invention, the speed of thespinner is substantially the same as in known systems. Fundamental tothe present design is the concept of polarization switching of theincident laser beam. In FIG. 6, a collimated beam of light 62 whichfeeds this scanner is circularly polarized in a clockwise rotation. Afirst quarter wave plate 64 positioned on a first spinner surface 66 toreceive the beam. A linearly polarized, first scan beam 68 is createdthrough the interaction of the 1/8th wave plate and transits apolarization sensitive beamsplitter (PSBS) 70 with an "S"" orientation.This first scan beam is received and reflected by a internal surface 72of the PSBS such that the reflected beam exits the spinner to follow thedirection of scanner rotation. The internal surface is polarizationsensitive such that incident light of select polarizations will betransmitted while other polarizations (e.g., "S" orientation) will bereflected. The PSBS surface reflects nearly 100% of the linearlypolarized light. There is also a lens 74 which focuses the first scanbeam before presentation to the substrate.

The first scan beam, therefore, is generated in a manner similar to thatdone in prior art systems and constitutes the initial beam generated bythe present system. For the second scan, an input (feed) beam to thescanner is polarization switched by 180° to circular/counterclockwise,as represented by beam 76 in FIG. 7. The first quarter-wave plate nowcreates a linearly polarized second scan beam 77 in a "P" orientationperpendicular to the S-beam. The light of the second scan beampropagates through the polarization sensitive beam splitter past theinternal surface with nearly 100% efficiency. Following the PSBS, thepolarization of the light is further rotated by 90 degrees by quarterwave plate 78 and reflected back by retroreflector 79. On return, theretroreflected beam is again polarization rotated by an additional 90degrees by the quarter wave plate to be polarized in the "S"orientation, as was the first scan beam. The PSBS internal surfacereflects the now S polarized second scan beam which transits a focusinglens 80 and presents the same to the substrate.

The present system takes advantage of the above spinner by including anaccousto-optical device 81 which receives switching signals from thecontroller to change the polarization of the input beam betweenclockwise and counterclockwise polarizations. Since the preferredencoder generates a once per revolution signal, the controller nowenables presentation of the modulated beam at two predetermined timesduring a each revolution of the spinner, as opposed to once perrevolution. Similar changes to the other components and systemparameters are accomplished as well, including a doubling of theadvancement speed in the slow scan direction.

Other examples of an optical return which can be substituted for theflat mirror of FIGS. 6 and 7 include a retroreflector, a roof prism or aroof mirror. Systems built in accordance with the present invention andwhich incorporate either a simple mirror or a retroreflector areburdened by the need for almost perfect alignment of the opticalcomponents which comprise the optic train.

Without such ideal alignment, the spinner will present the first andsecond beams to necessarily different positions on the substrate whenthey should, in fact, be superimposed (as determined without slow scanindexing). As shown in FIG. 8, there is seen two scan lines written on asubstrate. Lines 116, 118 are respectively generated by first and secondscan beams by a system with a simple mirror as an optical return andwith some error introduced. The lines clearly deviate from the idealpositions of nominal scanlines 120, 122 which would be produced by aperfect system.

The optic train which generates and guides the first and second scanbeams in the present system can (and typically does) have some degree ofmisalignment among the several optical components or inaccuracies in thecomponents themselves. Accordingly, the optic train, and the spinner inparticular, must be tolerant of first scan beam deviation from the inputoptic axis and the subsequently induced deviation from the preferredoptical path of the second scan beam, or simple variation of the secondscan beam from its preferred path. In certain situations the second scanbeam will be provided to the substrate at a location that is differentfrom the substrate position which receives the first scan beam even whena roof prism or mirror is employed as an optic return. As a result, the"written" scan lines will differ in position according to whetherwritten by the first or second scan beams. The tolerance for thismisalignment is extremely small; errors less than or equal to 20microinches are problematic.

The embodiment of the present invention as set forth with respect toFIG. 9 incorporates a roof prism or roof reflector as an optical returnof the second scan beam, and as such, can tolerate a wider range ofinput angles and displacements and yet still superimpose the first andsecond scan beams. In FIG. 9 there is schematically shown a simplifiedillustration of an alternative spinner 82 provided according to thepresent invention. The alternative spinner is substantially similar tothe spinner described above. A circularly polarized beam 84 is presentedalong input axis 86 to a quarter wave plate 88 and thereafter to apolarization sensitive beam splitter 90. The light either is reflectedfrom internal surface 92 or passes therethrough in dependence on thebeam's polarization.

A first scan beam 94 exits the polarization sensitive beam splitter 90along output axis 96 through lens 98. A second scan beam 100 passesthrough quarter wave plate 102 and enters an optical return 104 which isa roof prism in the Figure. The light is turned by reflection within theprism or roof reflector and presented again to quarter wave plate 102which rotates the polarization of the beam, resulting in the second scanbeam being reflected by the internal surface 92 and presented to lens106 along axis 108. Deficiencies in the optic train corresponding to anangular deviation 110 in the input beam from ideal coincidence withinput axis 86. As a result, scan lines vary from their respectivepreferred positions as written on a substrate surface.

In the embodiment of the present invention shown in FIG. 9, there isalso included apparatus 126, 127 for allowing lateral adjustment of thelenses 98, 106 relative to the output optic axes 96, 108. The apparatusare of a type known in the art and can be manually adjustable, as eitheror both of the lens positions can be adjusted during an initialalignment to remove any mispositioning of the scan lines relative to oneanother as written by the respective first and second beams, and therebycompensate for any errors from the optic train. In the alternative, thespinner may include an optical wedge or wedges 112 operated by controlapparatus 114 to remove error from the input beam.

It is further understood that only an adjustment mechanism cause scanline superposition in the slow or cross scan dimension is required.Adjustment during a scan can be provided effectively by means ofmodification to the pixel clocking electronics.

Spinners which have only simple reflectors (e.g. planar, reflectivesurfaces) are burdened by the need for dynamic compensation of scan lineposition errors. These systems must dynamically compensate for errors"on the fly" (as the scan line is written), since the optics of thesespinners do not allow for a single correction to be made which is validfor every pixel in the scan line. Accordingly, compensation apparatusmust be programmed with the appropriate magnitude of compensation foreach pixel position in a scan line written by each of the first andsecond scan beams.

FIGS. 13-15 graphically show the computed effect of error in the scanlines produced by the first and second scan beams with the embodiment ofthe present invention shown in FIG. 1 as compared with that of FIG. 9.The error can be the result of input beam misalignment, a mispositioningof one or more optic train elements, defects therein or combinationsthereof.

In FIG. 13, error is deliberately introduced into the first or primaryscan in a system as provided by the present invention and is manifestedas bow error (curve 170). The bow error is a function of spinnerrotation angle from 0 to 180 deg. and has a magnitude of plus or minusapproximately 0.22 mm. Curve 172 is shown in FIG. 14 and results in asystem in which a simple mirror is used as the optic return, while asystem which uses a roof mirror or prism yields curve 174 in FIG. 15. Acomparison of curves 172 and 174 reveals that only the system with theroof mirror produces in the second scan line the same error, both inmagnitude and sign, as was introduced into the first.

Accordingly, a simple adjustment to remove the error from the first scanbeam will remove the error from the second scan beam. Apparatus toremove the error is selected in dependence on the application. In asystem that employs a cube/beamsplitter and roof mirror, a wedge prismmay be inserted about the spin axis corotating with the assembly androtated to remove the error, as noted above. The compensation apparatusdescribed above is also preferred in Wollaston systems describedhereinafter.

Referring now to FIG. 10, there is shown a simplified schematicillustration of a alternative embodiment 128 of the present inventioncharacterized by a Wollaston prism 130. The alternative Wollaston systemalso employs polarization switching and a polarization sensitive opticalcomponent. As with other embodiments, an incident beam 132 of lighthaving either a left or right circular polarization is presented througha quarter wave plate 134 producing a beam of one linear polarization,(S) as an example. As the light transits the Wollaston prism, a firstscan beam 136 is generated which is displaced from axis 138 by adeviation angle 141. Light in a second scan beam 140 with thecomplimentary (P) polarization proceeds down a second path with an equaland opposite displacement angle from the optic axis. As seen in theschematic, sectioned illustration of the prism in FIG. 11, the beams142, 144 of different polarization transiting a Wollaston prism 130 aredeflected an equal amount from the optic axis 145.

Following the Wollaston prism, is a lens 146 centered about the axis andcommon to both polarization paths. The lens focuses the collimated lightto two separate foci 148, 150. In the embodiment of FIG. 10, the lens isof moderate complexity because of the off axis performance requirementsbut is otherwise of conventional design. A sectioned schematicillustration is found in FIG. 12 showing elements 152-156 that comprisethe lens providing a scan beam 158 to a substrate 160.

A double mirror 162 is also shown in FIG. 10 that has an "ax blade"geometry and which is positioned to receive the focused scan beams fromthe lens. Each beam is presented to a respective surface 164, 166 of themirror to fold out the beam towards the cylindrical imaging surface thatholds the substrate. In other embodiments, the lens structure may bedeleted, assuming that the "ax mirror" or equivalent optics containssome optical power to bring the beams into focus at the cylinder'ssurface.

The scanning system provided by this embodiment presents severaladvances over some of the other embodiments of the present inventiondescribed hereinabove. The Wollaston system is inherently symmetrical.In the presence of alignment errors to the scan assembly, the scan lineswhich are produced are exactly equal. The two complimentary beam pathsare collinear and are superimposed on the surface of the drum, assumingthat the drum is not moved in the slow scan direction. In other words,the input beam is now parallel to the spin axis. In the event of amisalignment within the rotating scanner, for example ax blade mirrorswhich are located at slightly different angles relative to the spinaxis, compensation is straightforward and is provided by shifting thescan lens laterally in its position with respect to the spin axis or byalignment of a co-rotating wedge prism.

In addition, the Wollaston system of FIG. 10 avoids spinning of thefocusing lenses off the input axis, and thereby avoids all of thechallenges of mechanical stress and stress induced optical birefringencewhich would otherwise occur. As a result, the system can be created tospin at higher speeds than would otherwise be possible in a system withoptical components located off axis. Moreover, the Wollaston systempresents fewer parts than does other embodiments of the presentinvention, and the tolerance requirements on these parts are generallylower than that of the above described embodiments.

Similar systems may be built using a Rochon prism in place of theWollaston prism as shown in FIG. 16. As is known, a Rochon prism 176which receives a beam 178 will deflect light of a selected polarization.A scan beam 180 of a first polarization will exit the prism along opticaxis 182 directly, with the second scan beam 184 presented at an anglethereto. A Glan or Glan-Thompson prism can also be used to construct asystem of the present type without substantial modification to thesystems described hereinabove. Those skilled in the art will note that aGlan-Thompson prism is similar to a Nicol prism which produces plainpolarized light, but has its internal faces normal to the optic axis.

Referring now to FIG. 17 there is shown in diagrammatic form anillustration of an alternative embodiment 186 of the present inventioncharacterized by the simultaneous presentation of dual scan beams. Incomparison to the embodiments described hereinabove, the dual scansystem of FIG. 17 has two light sources 188 and 190 which are bothcircularly polarized but whose polarization have the opposite sense(e.g., right hand vs. left hand circular polarization). Both beams arecombined at beam combiner 192 for presentation along scan axis 194. Bothbeams are then presented simultaneously to scanner 196 which issubstantially as described hereinabove with respect to FIG. 9. Anadjustment mechanism to 198 may also be included to perform the samefunctions as noted above.

With the embodiment of FIG. 17, each beam 200 and 202 maps directly backto its laser modulator and data stream. Consequently, the number ofscans per revolutions has doubled. The embodiments described previouslygenerate two scans per revolution whenever the active scanning is lessthan π radians (180°). The embodiment of FIG. 17 no longer has the limitof 180°. However, two modulators as well as two optical sources (lasers)and beam combining optics are required with this embodiment of thepresent invention.

The Wollaston system set forth above suffers from several drawbackswhich can affect system performance. These include an asymmetry indeviation angles of the beams output from the Wollaston prism. An inputbeam which traverses a path at a slight angle to optic axis will exit aWollaston prism at an angle whose magnitude is dependent on beampolarization. In practice, typical deviation angles are -9.7° and+10.9°. These may be balanced to a mean separation of 10.3° with theaddition of an optical wedge. However, this configuration does not solveother problems which afflict the Wollaston system.

The Wollaston system also possess an asymmetry in the angularmagnification. The input beam is received by the prism at a small angleto the optic axis. This small input angle is typically due to a residualalignment error between the optical chassis and the scanner (e.g. 0-5arc minutes). Beam magnification is different for each polarization. Inorder for the system as a whole tolerate alignment errors, the Wollastonsystem needs to be highly symmetrical. For an input angular error of Sarc minutes, the Wollaston prism will generate a constant tilt error inthe output beams within 0.2 arc seconds, in addition to the input tilt.The standard Wollaston prism has a tilt response of about 1 arc minuteto a 5 arc minute input (or 300 times the tilt error). Moreover, theWollaston prism has an exit pupil, representing the axial tilt position,which is at unequal planes for the two cases.

All of these problems are effectively addressed in a modified Wollastonprism system 204 of FIG. 18, also referred to as a Straayer prismassembly. The system 204 is substantially the same as shown with respectto FIG. 10, but includes a prism assembly 206 comprised of first andsecond prisms 208, 210 substantially identical, but with crystal axes212, 214 oriented orthogonal to one another. The crystallographic axisof the first prism extends out from the page, while that of the secondprism is lengthwise in the Figure. The prisms each have base angles of67 deg, 20' plus/or minus 10', with an apex angle of 48 deg, 50'plus/minus 10'. In the embodiment of FIG. 18, the prisms are cementedtogether so the assembly has a base of 12 mm plus/minus 0.2 mm and aheight of 13.2 mm. The specifications set forth above are exemplary of aselected aperture and wavelength (488 mm).

There is also a glass (SF57) window 216 having a known anti-reflectioncoating. The window is 6 mm plus/minus 0.25 mm in thickness and 20 mmplus/minus 0.25 mm in diameter and is placed in the system to receive aninput optical beam prior to presentation of the same to the prismassembly. The index, thickness, and tilt angle of this window are chosento force the beam deviations to occur from a common exit pupil plane.

The design of this modified Wollaston system has several distinguishingcharacteristics. The beam has an angle of incidence on first prismsurface which is equal to the mean angle of exit or average anglebetween the two deviation beams. The angle of incidence in the systemshown in FIG. 18 is approximately 22.5°. The prism angle of thedeviation prisms being nearly equal (approximately 48°). The addition ofa plane parallel deviation window brings the exit pupils of the two beamdeviation cases into coincidence.

FIGS. 19 and 20 illustrate the performance of the modified Wollastonsystem of FIG. 18 when receiving first and second beams 218 and 220,respectively, of different polarization. In FIGS. 19 and 20, as in FIG.12 the beams traverse from right to left in contrast to the severalother Figures.

In FIG. 19, input beam 218 is presented first through window 216 beforetransiting prism assembly 206. The beam is then deflected downward inthe Figure to be received by a first surface of the double mirror as inthe system of FIG. 12. The double mirror is not shown in FIGS. 19 or 20,but is substantially as indicated with respect to the Wollaston prismembodiment of FIG. 10. In FIG. 20, the second beam 220 which has theopposite circular polarization as compared with beam 218 is presentedthrough the same window 216 and prism assembly but now is deflectedupwards in the Figure for presentation to a second surface of the doublemirror.

Similarly, although the invention has been shown and described withrespect to a preferred embodiment thereof, it would be understood bythose skilled in the art that other various changes omissions andadditions thereto may be made without departing from the spirit andscope of the present invention.

I claim:
 1. A scanning optical system receiving a substantiallycircularly polarized optical beam, said system comprising:a modulatormeans for providing modulation to said circularly polarized optical beamin response to received modulator control signals; a curved platen forreceiving a substrate; a raster scanning means responsive to advancementcontrol signals for advancing, relative to said substrate, an opticalbeam across a substrate surface in a first direction forming a scan lineand for cooperatively advancing, relative to said substrate surface,said circularly polarized optical beam in a second directionsubstantially perpendicular to said first direction; a means forgenerating first and second optical scan beams respectively polarized infirst and second polarization directions in response to polarizationcontrol signals; an encoder means for generating encoder signalsindicative of the angular position of a spinner about a scan axis andconsequently indicative of the optical beam along a current scan line; acontroller, receiving said encoder signals, for generating saidadvancement control signals and said modulator control signals, saidcontroller further providing said optical beam polarization switchingsignals in dependence on said encoder signals such that said opticalbeam polarization is switched between said first and second polarizationdirection after the completion of the current scan line; said spinnerreceiving said first and second optical scan beams and includinga firstquarter wave plate, receiving said optical beam, for providing linearlypolarized first and second scan beams; a polarization sensitivebeamsplitter for presenting each of said first and second linearlypolarized scan beams respectively along a one of two scan beam outputaxes displaced from each other and selected in dependence upon said scanbeam polarization; a focusing means for receiving and focusing saidbeamsplitter output beams along substantially parallel axes onto saidsubstrate surface; and an optical deflection means for presenting saidfirst and second linearly polarized scan beams in opposed directions,thereby generating two scan beams per beam splitter revolution aboutsaid scan axis.
 2. The scanning optical system of claim 1 wherein saidoptical deflection means further comprises an ax-blade mirror.
 3. Thescanning optical system of claim 1 wherein said polarization sensitivebeamsplitter further comprises a Wollaston prism.
 4. The scanning systemof claim 3 further combining a second quarter wave plate positioned toreceive said first and second scan beam subsequent to said WollastonPrism, thereby ensuring said scan beams presented to said substrate arecircularly polarized.
 5. The optical system of claim 3 wherein saidWollaston prism comprises a prism assembly of first and second prismseach having respective crystallographic axes, oriented orthogonal to oneanother; anda glass window receiving said optical beam prior topresentation to said Wollaston prism, said window positioned at aselected angle of tilt to said input beam optic axes, thereby bringingexit pupils of said first and second optical scan beams intocoincidence.
 6. The optical system of claim 5 wherein said first andsecond prisms are cemented together and each prism having a base angleof approximately 67 degrees, 20 minutes with an apex angle ofapproximately 48 degrees, 50 minutes, said window tilt angleapproximately equal to 30 degrees.
 7. The optical system of claim 6wherein said first and second prisms each have base angles ofapproximately 67 deg, 20' plus/or minus 10', with an apex angle of 48deg, 50' plus/minus 10'.
 8. The scanning optical system of claim 1wherein said polarization sensitive beamsplitter further comprises aRochon prism.